[ruby-core:102706] [Ruby master Bug#17669] An exception still breaks monitor state and causes deadlock in 2.6.7

Issue #17669 has been reported by yoheimuta (Yohei Yoshimuta).

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Bug #17669: An exception still breaks monitor state and causes deadlock in 2.6.7
https://bugs.ruby-lang.org/issues/17669

* Author: yoheimuta (Yohei Yoshimuta)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.6.7p153 (2021-01-31 revision 67892) [x86_64-darwin19]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
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`lib/monitor.rb` provides Monitor.

However, its state handling is weak for interrupts caused by Thread.kill for example timeout libraries
even after introducing some uses of Thread.handle_interrupt at https://bugs.ruby-lang.org/issues/15992.

Actually, timeout exception may happen everywhere.
If it raised when the thread is executing right before the begin block,

```rb
  def mon_synchronize
    # Prevent interrupt on handling interrupts; for example timeout errors
    # it may break locking state.
->  Thread.handle_interrupt(Exception => :never){ mon_enter }
    begin
      yield
    ensure
      Thread.handle_interrupt(EXCEPTION_NEVER){ mon_exit }
    end
  end
```

it breaks the state of the monitor and it causes deadlock.

I can confirm that this happens either in 2.6.7 head and 2.6.6 release.

```
/bin/bash -c \
"date; ruby -v; ruby reproducible.rb; tail -n 10 /tmp/tmp.txt; date;" | tee ruby:2.6.7-macosx.log
```

```
docker run -it --rm -v `pwd`:`pwd` -w `pwd` ruby:2.6.6-alpine3.13 /bin/ash -c \
"date; ruby -v; ruby reproducible.rb; tail -n 10 /tmp/tmp.txt; date;" | tee ruby:2.6.6-alpine3.13.log
```

Technically, 2.5.8 is also reproducible because it shares the same releated code. 
Incidentally, this doesn't happen in either 2.7.2 and 3.0.0 because [the monitor was reimplemented in C](https://bugs.ruby-lang.org/issues/16255).

Our production busy puma servers have suffered this weakness susceptible to timeouts, which frequently causes completely hung worker threads in a process.

The commit https://github.com/ruby/ruby/pull/4204/commits/e99c823f16918677b823255c44142910e02922c1 should fix this issue.

---Files--------------------------------
reproducible.rb (1.71 KB)
ruby_2.6.6-alpine3.13.log (12.8 KB)
ruby_2.6.7-macosx.log (3.73 KB)


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